Even though people can understand the existence and oneness of Allah with their minds, they cannot comprehend His names and attributes and cannot know how to worship Him. They cannot comprehend the affairs related to the hereafter and the responsibility in the hereafter. They are in need of being informed about those issues by Allah.
Allah sent prophets in order to meet those needs of people. When people did not know anything, they learned everything, every art and skill from prophets. Prophets also taught people the ways of being happy and living peacefully in the world and in the hereafter, the principles of getting on well with one another and the rules of ethics and good manners.
Imam Ghazali states the following regarding the issue:
“Man cannot discriminate between lethal poison and useful medicine and he needs a doctor or chemist; he cannot know the beliefs and the deeds that will save him in the world and in the hereafter on his own, either. He cannot discriminate between the things that will lead him to disasters from the things that will lead him to happiness. He certainly needs a guide. And that guide is a prophet.” (Ghazali, Ihya trns., I, 291.)
Badiuzzaman says the following regarding the issue:
“The divine power, which does not leave ants without a leader and bees without a queen, will definitely not leave human beings without a prophet.” (Sözler, 792)
Nothing in the universe, especially in the realm of living beings has been left purposeless, dutiless, systemless and even guideless. It is possible to see it in pieces or as a whole. Neither the bees are queenless, nor ants are leaderless. (Abdullah Aymaz, Peygamberlik, p.7.)
Since guides are not neglected for all of the other creatures, is it possible for man, who is regarded as superior by both the religion and science, to be left without prophets, who are essential guides? That is, is it possible to imagine the humankind without prophets or books?” (ibid)
We can list the need for prophets as follows:
1. Ensuring Order and Justice in Society:
The law and order in the community, the peace and happiness of the individuals, the prevention of oppression and injustice depend on the existence of the institution of prophethood. The issue is explained as follows in İşâratü’l-İ’câz:
“Man has been created with a strange, subtle nature distinguishing him from all other living beings and making him an exception to them. By virtue of this, there have arisen in him the desire to choose, and the wish for the most beautiful things and for fine decoration, and a natural desire to have a livelihood and position befitting humanity.
For his food, dress and accommodation – the for which arises due to the above-mentioned desires – to be prepared proficiently, man has need of multiple arts and crafts, but he is incapable of practicing all of these on his own. He needs, therefore, to mix with his fellow-men and to co-operate with them, and for all of them to assist each other and exchange the fruits of their labor.
However, since so as to allow men’s progress by means of the mainspring of the will the All-Wise Maker placed no innate limits on the human powers of appetite, anger and intellect – as with the animals and their limited powers – tyranny and aggression have arisen. And since no limit has been placed on them, the powers tend to aggression and the human community is in need of justice when exchanging the fruits of its labor. However, because the intellects of single members of society are incapable of comprehending justice, the human race as a whole needs a general or universal intellect to establish from which all may benefit. And that intellect is the universal law, and that law is the Shari’a.
Then in order to preserve the effectiveness of the Shari’a and its enforcement, a law-maker is necessary, someone to lay claim to it and to promulgate it, an authority, and that is the Prophet (pbuh). Then, in order to perpetuate his outward and inward domination over people’s minds and hearts, the Prophet needs to be eminently superior both physically and spiritually and morally, and in conduct and appearance, and by nature and in character. He is in need, too, of proof of the strength of his relation with the Creator, and such a proof is his miracles.”(İşâratü’l-İ’câz, 92-93)
2. Teaching man the reason why he was created and informing him about religious decrees:
As it is stated in the Quran the purpose and wisdom of man’s creation and being sent down to the world is to know his Creator, to believe in Him and to worship Him. (az-Zariyat, 56; see also: Ayatu’l-Kubra, 8)
The mind cannot understand the aim of the creation on its own. Even if we accept that man can understand the existence of God Almighty with his mind as some scholars of Kalam claim, he cannot understand how to worship Him and how to please Him on his own. Therefore, prophets are necessary.
God Almighty informed His distinguished servants that He chose among human beings about all of His orders and prohibitions. They practiced those decrees themselves first and then conveyed them to other people, making them know the Creator of the universe. (For detailed information, see: Sözler, 56)
3. Blocking the Ways of Objection to Allah:
It is stated in the Quran that each tribe and nation was definitely sent a prophet that gave them the glad tidings of the mercy of Allah and warned them about the torture of Allah.
One of the reasons why each nation was sent a prophet like that is stated as follows in the Quran:
“Messengers who gave good news as well as warning, that mankind, after (the coming) of the messengers should have no plea against Allah: for Allah is Exalted in Power, Wise.” (an-Nisa, 165)
Another verse is as follows:
“If (We had) not (sent thee to the Quraysh)― in case a calamity should seize them for (the deeds) that their hands have sent forth, they might say: “Our Lord! why didst Thou not send us a messenger? We should then have followed the Signs and been amongst those who believe!”(al-Qasas, 47)
An important reason of the existence of the institution of prophethood is understood from those verses.